Tony Blair privately assured President George Bush in letters written a year before the invasion of Iraq that Britain would "be there" in any US-led attack on the country, it was revealed at the Chilcot inquiry today.

The disclosure came during sometimes sharp exchanges with Alastair Campbell, Blair's communications chief and close adviser, who described Gordon Brown, the then chancellor, as "one of the key ministers" Blair spoke to about Iraq.

• Said Britain should be "proud" of its role in the overthrow of Saddam Hussein.

• Said Blair tried to get the conflict with Iraq resolved "without a shot being fired".

Blair wrote "quite a lot of notes" to Bush in 2002 and their substance was not shared with the cabinet, Campbell made clear. Asked if the then foreign secretary, Jack Straw, knew their contents, Campbell replied: "I very much doubt if drafts went round the system … They were very frank." However, Campbell said they were discussed with Sir David Manning, Blair's foreign policy adviser.

He said the tenor of the letters was: "We share the analysis, we share the concern, we are going to be with you in making sure that Saddam Hussein is faced up to his obligations and that Iraq is disarmed." Campbell added: "If that cannot be done diplomatically and it is to be done militarily, Britain will be there. That would be the tenor of the communication to the president."

The letters Blair wrote to Bush have been passed to the Chilcot inquiry. It has not given any indication about whether it will publish them.

Campbell was responding to persistent questioning from Sir Roderic Lyne, a member of the inquiry panel and a former ambassador. Lyne referred Campbell to a leaked document in which Manning, on a trip to Washington in March 2002, a year before the invasion, told Blair he had underlined Britain's position to ­Condoleezza Rice, Bush's national security adviser.

"I said you [Blair] would not budge in your support for regime change, but you had to manage a press, a parliament, and a public opinion which is very different than anything in the States," Manning wrote.

Responding to Lyne's question, Campbell said: "The prime minister's overall approach was saying 'there's going to be disarmament. We're going to do our level best to get that through the diplomatic route, without a single shot being fired but, if push comes to shove and the diplomatic route fails, Britain would see it as its responsibility and its duty to take part in military action'."

Blair was determined to disarm Saddam, Campbell said. Blair's message to the US in April 2002 was he would try to do it through UN resolutions. ­However, "if the only way is regime change through military action then the British government will support the American government", Campbell said, describing Blair's view.

The inquiry has also heard from ­senior British diplomats that regime change was being discussed by Blair in the US in 2002 even though, according to leaked documents, Lord Goldsmith, the attorney general, warned the PM that military action aimed at regime change, as opposed to disarmament, would be unlawful.

Campbell stoutly defended the ­September 2002 Iraqi weapons dossier which stated that Saddam was continuing to build up a nuclear, chemical, and biological weapons programme – claims that were shown to be without foundation after the invasion. He insisted that Sir John Scarlett, then chairman of the Joint Intelligence Committee, was adamant throughout that he was "100% in charge" of the process of drawing up the dossier.

"At no time did I ask him to beef up, to override, any of the judgments that he had," Campbell told the inquiry. "John Scarlett said to me 'This is a document the prime minister is going to present to parliament, there are massive global expectations around it, and I need a bit of presentational support,' and that is what I gave him."

At no time did Scarlett or intelligence officers question the contents of the dossier, said Campbell.

Campbell on occasions sharply criticised the British media and played down any influence he had over journalists. Asked about the notorious claim in the weapons dossier that Iraq could deploy chemical and biological weapons within 45 minutes he said it had only been given "iconic" status by the press.

Asked if it could have been made clear that the claim only ever applied to battle­field weapons rather than longer range missiles, Campbell replied: "Obviously, but it's not that big a point."

He disclosed that the international development secretary Clare Short, who subsequently resigned over the war, had been excluded from discussions on the aftermath of the conflict because of fears of leaks.

"I think in an ideal world the secretary of state for international development would, should and could have been involved in all those discussions," he said. "It was no secret that she was very difficult to handle at times. I think sometimes the military found her approach to them difficult to deal with."

Nicholas Watt: Now we know that while Tony Blair may have been genuine in his efforts to pursue a diplomatic route he was so determined to remove Saddam Hussein that he was committed from an early stage to a military route